The mix of a sound system varies with the position of the listener in the venue. Ideally, sound systems are mixed in the middle of an audience. However, this position is often not available to the sound engineer because of the amount of space taken by the audio gear which reduces the number of audience seats thereby leading to reduced ticket revenue. Often, the sound gear is placed next to the stage or on the stage and operated by one of the musicians. Even when the sound system is mixed from a non-ideal position, it is still necessary to know the content of the mix in the audience away from the gear. Sometimes headphones are used to try to listen to the mixing console's output, but the stage volume is often too loud to effectively hear the mix in the headphones.